In the field of LED drivers for offline applications such as retrofit lamps and new lamps or modules, solutions are demanded to cope with high efficiency, high power density and high power factor among other relevant features. While practically all existing solutions comprise one or another requirement, it is essential that the proposed driver circuit properly conditions the form of the mains energy into the form required by the LEDs while remaining in compliance with present and future power mains regulations. It is of critical importance to control the amount of power delivered to the lamps to control the brightness of the lamps, while having a high efficiency and reduced power loss in the power converter. To control the amount of power delivered to the lamps, phase cut dimming is one option having a high efficiency and a low power loss. If driver devices are used including a phase cut dimmer, the lamps derive the electrical power from the phase cut mains voltage and have to recover the phase cut position, in order to set the power level of the lamp accordingly. Trailing edge phase cut dimmers, which are preferably used, do not always provide a voltage step with a significant edge, which is easy to detect due to the filter capacitors across the lamp and across the dimmer. Therefore, the lamps are provided with a bleeder circuit having one or more bleeder resistors to drain the charged capacitor, in order to verify that the dimmer is turned off. However, the bleeding current increases the power loss of the lamps.
WO 2010137002 A1 discloses a phase cut dimmer device for driving an LED unit, wherein the LED unit comprises a bleeder circuit to adjust the rectified phase cut input voltage. The bleeder circuits comprise detection means to detect the voltage drop at two predefined voltage levels to activate one of the two bleeder circuits. Detection of the phase angle of the phase cut voltage in an accurate manner is not possible with this bleeder circuit.